108 PLANT STKUCTUEES 



however, it shows an interesting evolution from its sim- 

 plest condition in the Liverworts to its most complex con- 

 dition in the true Mosses. 



In the Liverworts the spore develops a flat thallus body, 

 one plate of cells or more in thickness, which generally 

 branches dichotomously (see § 29) and forms a more or less 

 extensive body (Fig. 92). This thallus is the gametophyte, 

 there being no differentiation into protonema and leafy 

 branch. 



In the simpler Liverworts the sex organs (antheridia 

 and archegonia) are scattered over the back of this thallus 

 (Fig. 92). In other forms they become collected in certain 

 definite regions of the thallus. In other forms these defi- 

 nite sexual regions become differentiated from the rest of 

 the thallus as disks. In other forms these disks, bearing 

 the sex organs, become short-stalked, and in others long- 

 stalked, until a regular branch arises from the thallus 

 body (Figs. 96, 97). This erect branch, bearing the sex or- 

 gans, is, of course, a gametophore, but it is leafless, the 

 thallus body doing the chlorophyll work. 



In the Sphagnum Mosses the spore develops the same 

 kind of flat thallus (Fig. 104), but the gametophore be- 

 comes leafy, sharing the chlorophyll work with the thallus. 

 In the true Mosses most of the chlorophyll work is done by 

 the leafy gametophore, and the flat thallus is reduced to 

 branching filaments (the protonema) (Fig. 102). 



The protonema of the true Mosses, therefore, corre- 

 sponds to the flat thallus of the Liverworts and Sphagnum, 

 while the leafy branch corresponds to the leafless gameto- 

 phore found in some Liverworts. It also seems evident 

 that the gametophore was originally set apart to bear sex 

 organs, and that the leaves which appear upon it in the 

 Mosses are subsequent structures. 



